Bispecific antibodies (biAbs) that exert cytotoxicity by binding to tumor cells with one arm and by simultaneously recruiting and activating tumor cell-lysing endogenous immune cells with the other arm are an emerging category of next-generation antibody drugs for cancer therapy. Our research team will develop and deliver conceptually novel chemically programmed biAbs that recognize tumor cells with a variable small molecule component and that recruit and activate T cells and NK cells with a generic antibody component. Chemically programmed biAbs are more versatile than conventional biAbs as they only require the cloning, expression, and purification of a single protein. Further, to target a variety of different tumor cell surface antigens, chemically programmed biAbs can make use of a wealth of small molecules derived from chemical libraries or from structure-based design campaigns, linking advances in both immunology and chemistry for the benefit of cancer patients. The proposed study will rigorously test the hypothesis that chemically programmed biAbs can recruit and activate T cells and NK cells that selectively and potently kill tumor cells n vivo. Specifically, we will develop and deliver two entirely different molecular formats of the generic antibody component and then chemically program biAbs to selectively target folate receptor 1 (FOLR1). FOLR1 was chosen as a prototype as this tumor cell surface antigen is a clinically investigated target for both small molecules and monoclonal antibodies in ovarian and lung cancer and in other devastating solid malignancies. The two molecular formats that will be interrogated are based on the reactive selenocysteine (Sec) and the reactive lysine (Lys) technologies that we developed for molecularly defined chemical programming of antibodies. In Aim 1 we will generate and validate chemically programmed (FOLR1 x CD3) and (FOLR1 x NKG2D) biAbs based on a single antibody module in Fab format with an engineered C-terminal Sec. In Aim 2 we will generate and validate chemically programmed (FOLR1 x CD3) and (FOLR1 x NKG2D) biAbs based on a dual antibody module in DART (Dual-Affinity Re-Targeting) format that displays a single reactive Lys residue. Chemically programmed biAbs in these two molecular formats will be analyzed and compared for their ability to recruit and activate T cells (via CD3) and NK cells (via NKG2D) to direct killing of FOLR1-expressing tumor cells. Finally, in Aim 3, we will test the efficacy and safety of our chemically programmed biAbs in immunocompromised mice engrafted with both human effector and target cells. Collectively, our campaign will deliver both novel concepts and constructs for next-generation antibody drugs that are explicitly designed for broad utility in cancer therapy.